There are several reasons why this may have been used. It’s a common kitchen item and doesn’t appear suspicious. Pressure cooker bombs are also fairly simple to make. And when the bomb explodes, the metal pot becomes shrapnel.

When asked if the powder is actually a calling card as to who made the bomb, Horrace said, “It could be. What we’ve seen in a lot of explosions throughout the years and a lot of the bombings, the bombers use a combination of high explosives and low explosives, black powder and smokeless powder, and there are different signatures for the different individuals involved.”

The number of injured from Monday’s bombs continue to grow and at least one doctor treating those patients reported pulling nail-like items and pellets from some of the wounded. Experts said the bombs contained shrapnel — likely BBs or ball bearings and nails.

“The only purpose to put shrapnel inside a device is to injure people and more people,” Horrace said.

Another challenge for investigators is some if not many of the items used in these bombs are common household items.

“Every bit of evidence will go towards trying to build that story and figure out who is responsible for making the device, where the items were that supported making the device and where these things were purchased,” Horrace said.

According to Department of Homeland Security experts, fighters are taught in Afghan terrorist training camps to use pressure cookers to hold bombs, because they are very common in that country and don’t draw attention when left in the open.